1. Field of the Invention
The present invention relates generally to the gas enrichment of a fluid and, more particularly, to the detection of bubbles in the gas-enriched fluid.
2. Background of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Gas-enriched fluids are used in a wide variety of medical, commercial, and industrial applications. Depending upon the application, a particular type of fluid is enriched with a particular type of gas to produce a gas-enriched fluid having properties that are superior to the properties of either the gas or fluid alone for the given application. The techniques for delivering gas-enriched fluids also vary dramatically, again depending upon the particular type of application for which the gas-enriched fluid is to be used.
Many commercial and industrial applications exist. As one example, beverages may be purified with the addition of oxygen and carbonated with the addition of carbon dioxide. As another example, the purification of wastewater is enhanced by the addition of oxygen to facilitate aerobic biological degradation. As yet another example, in fire extinguishers, an inert gas, such as nitrogen, carbon dioxide, or argon, may be dissolved in water or another suitable fluid to produce a gas-enriched fluid that expands on impact to extinguish a fire.
While the commercial and industrial applications of gas-enriched fluids are relatively well known, gas-enriched fluids are continuing to make inroads in the healthcare industry. Oxygen therapies, for instance, are becoming more popular in many areas. A broad assortment of treatments involving oxygen, ozone, H2O2, and other active oxygen supplements has gained practitioners among virtually all medical specialties. Oxygen therapies have been utilized in the treatment of various diseases, including cancer, AIDS, and Alzheimer's. Ozone therapy, for instance, has been used to treat several million people in Europe for a variety of medical conditions including excema, gangrene, cancer, stroke, hepatitis, herpes, and AIDS. Such ozone therapies have become popular in Europe because they tend to accelerate the oxygen metabolism and stimulate the release of oxygen in the bloodstream.
Oxygen is a crucial nutrient for human cells. It produces energy for healthy cell activity and acts directly against foreign toxins in the body. Indeed, cell damage may result from oxygen depravation for even brief periods of time, and such cell damage can lead to organ dysfunction or failure. For example, heart attack and stroke victims experience blood flow obstructions or divergence that prevent oxygen in the blood from being delivered to the cells of vital tissues. Without oxygen, these tissues progressively deteriorate and, in severe cases, death may result from complete organ failure. However, even less severe cases can involve costly hospitalization, specialized treatments, and lengthy rehabilitation.
Blood oxygen levels may be described in terms of the concentration of oxygen that can be achieved in a saturated solution at a given partial pressure of oxygen (pO2). Typically, for arterial blood, normal oxygen levels, i.e., normoxia or normoxemia, range from 90 to 110 mmHg. Hypoxemic blood, i.e., hypoxemia, is arterial blood with a pO2 less than 90 mmHg. Hyperoxemic blood, i.e., hyperoxemia or hyperoxia, is arterial blood with a pO2 greater than 400 mmHg, but less than 760 mmHg. Hyperbaric blood is arterial blood with a pO2 greater than 760 mmHg. Venous blood, on the other hand, typically has a pO2 level less than 90 mmHg. In the average adult, for example, normal venous blood oxygen levels range generally from 40 mmHg to 70 mmHg.
Blood oxygen levels also may be described in terms of hemoglobin saturation levels. For normal arterial blood, hemoglobin saturation is about 97% and varies only as pO2 levels increase. For normal venous blood, hemoglobin saturation is about 75%. Indeed, hemoglobin is normally the primary oxygen carrying component in blood. However, oxygen transfer takes place from the hemoglobin, through the blood plasma, and into the body's tissues. Therefore, the plasma is capable of carrying a substantial quantity of oxygen, although it does not normally do so. Thus, techniques for increasing the oxygen levels in blood primarily enhance the oxygen levels of the plasma, not the hemoglobin.
The techniques for increasing the oxygen level in blood are not unknown. For example, naval and recreational divers are familiar with hyperbaric chamber treatments used to combat the bends, although hyperbaric medicine is relatively uncommon for most people. Since hemoglobin is relatively saturated with oxygen, hyperbaric chamber treatments attempt to oxygenate the plasma. Such hyperoxygenation is believed to invigorate the body's white blood cells, which are the cells that fight infection. Hyperbaric oxygen treatments may also be provided to patients suffering from radiation injuries. Radiation injuries usually occur in connection with treatments for cancer, where the radiation is used to kill the tumor. Unfortunately, at present, radiation treatments also injure surrounding healthy tissue as well. The body keeps itself healthy by maintaining a constant flow of oxygen between cells, but radiation treatments can interrupt this flow of oxygen. Accordingly, hyperoxygenation can stimulate the growth of new cells, thus allowing the body to heal itself.
Radiation treatments are not the only type of medical therapy that can deprive cells from oxygen. In patients who suffer from acute myocardial infarction, for example, if the myocardium is deprived of adequate levels of oxygenated blood for a prolonged period of time, irreversible damage to the heart can result. Where the infarction is manifested in a heart attack, the coronary arteries fail to provide adequate blood flow to the heart muscle. The treatment for acute myocardial infarction or myocardial ischemia often involves performing angioplasty or stenting of vessels to compress, ablate, or otherwise treat the occlusions within the vessel walls. In an angioplasty procedure, for example, a balloon is placed into the vessel and inflated for a short period of time to increase the size of the interior of the vessel. When the balloon is deflated, the interior of the vessel will, hopefully, retain most or all of this increase in size to allow increased blood flow.
However, even with the successful treatment of occluded vessels, a risk of tissue injury may still exist. During percutaneous transluminal coronary angioplasty (PTCA), the balloon inflation time is limited by the patient's tolerance to ischemia caused by the temporary blockage of blood flow through the vessel during balloon inflation. Ischemia is a condition in which the need for oxygen exceeds the supply of oxygen, and the condition may lead to cellular damage or necrosis. Reperfusion injury may also result, for example, due to slow coronary reflow or no reflow following angioplasty. Furthermore, for some patients, angioplasty procedures are not an attractive option for the treatment of vessel blockages. Such patients are typically at increased risk of ischemia for reasons such as poor left ventricular function, lesion type and location, or the amount of myocardium at risk. Treatment options for such patients typically include more invasive procedures, such as coronary bypass surgery.
To reduce the risk of tissue injury that may be associated with treatments of acute myocardial infarction and myocardial ischemia, it is usually desirable to deliver oxygenated blood or oxygen-enriched fluids to the tissues at risk. Tissue injury is minimized or prevented by the diffusion of the dissolved oxygen from the blood to the tissue. Thus, in some cases, the treatment of acute myocardial infarction and myocardial ischemia includes perfusion of oxygenated blood or oxygen-enriched fluids. The term “perfusion” is derived from the French verb “perfuse” meaning “to pour over or through.” In this context, however, perfusion refers to various techniques in which at least a portion of the patient's blood is diverted into an extracorporeal circulation circuit, i.e., a circuit which provides blood circulation outside of the patient's body. Typically, the extracorporeal circuit includes an artificial organ that replaces the function of an internal organ prior to delivering the blood back to the patient. Presently, there are many artificial organs that can be placed in an extracorporeal circuit to substitute for a patient's organs. The list of artificial organs includes artificial hearts (blood pumps), artificial lungs (oxygenators), artificial kidneys (hemodialysis), and artificial livers.
During PTCA, for example, the tolerable balloon inflation time may be increased by the concurrent introduction of oxygenated blood into the patient's coronary artery. Increased blood oxygen levels also may cause the hypercontractility in the normally perfused left ventricular cardiac tissue to increase blood flow further through the treated coronary vessels. The infusion of oxygenated blood or oxygen-enriched fluids also may be continued following the completion of PTCA or other procedures, such as surgery, to accelerate the reversal of ischemia and to facilitate recovery of myocardial function.
Conventional methods for the delivery of oxygenated blood or oxygen-enriched fluids to tissues involve the use of blood oxygenators. Such procedures generally involve withdrawing blood from a patient, circulating the blood through an oxygenator to increase blood oxygen concentration, and then delivering the blood back to the patient. There are drawbacks, however, to the use of conventional oxygenators in an extracorporeal circuit. Such systems typically are costly, complex, and difficult to operate. Often, a qualified perfusionist is required to prepare and monitor the system. A perfusionist is a skilled health professional specifically trained and educated to operate as a member of a surgical team responsible for the selection, setup, and operation of an extracorporeal circulation circuit. The perfusionist is responsible for operating the machine during surgery, monitoring the altered circulatory process closely, taking appropriate corrective action when abnormal situations arise, and keeping both the surgeon and anesthesiologist fully informed. In addition to the operation of the extracorporeal circuit during surgery, perfusionists often function in supportive roles for other medical specialties to assist in the conservation of blood and blood products during surgery and to provide long-term support for patient's circulation outside of the operating room environment. Because there are currently no techniques available to operate and monitor an extracorporeal circuit automatically, the presence of a qualified perfusionist, and the cost associated therewith, is typically required.
Conventional extracorporeal circuits also exhibit other drawbacks. For example, extracorporeal circuits typically have a relatively large priming volume. The priming volume is typically the volume of blood contained within the extracorporeal circuit, i.e., the total volume of blood that is outside of the patient's body at any given time. For example, it is not uncommon for the extracorporeal circuit to hold one to two liters of blood for a typical adult patient. Such large priming volumes are undesirable for many reasons. For example, in some cases a blood transfusion may be necessary to compensate for the blood temporarily lost to the extracorporeal circuit because of its large priming volume. Also, heaters often must be used to maintain the temperature of the blood at an acceptable level as it travels through the extracorporeal circuit. Further, conventional extracorporeal circuits are relatively difficult to turn on and off. For instance, if the extracorporeal circuit is turned off, large stagnant pools of blood in the circuit might coagulate.
In addition to the drawbacks mentioned above, in extracorporeal circuits that include conventional blood oxygenators, there is a relatively high risk of inflammatory cell reaction and blood coagulation due to the relatively slow blood flow rates and large blood contact surface area of the oxygenators. For example, a blood contact surface area of about one to two square meters and velocity flows of about 3 centimeters/second are not uncommon with conventional oxygenator systems. Thus, relatively aggressive anti-coagulation therapy, such as heparinization, is usually required as an adjunct to using the oxygenator.
Finally, perhaps one of the greatest disadvantages to using conventional blood oxygenation systems relates to the maximum partial pressure of oxygen (pO2) that can be imparted to the blood. Conventional blood oxygenation systems can prepare oxygen-enriched blood having a partial pressure of oxygen of about 500 mmHg. Thus, blood having pO2 levels near or above 760 mmHg, i.e., hyperbaric blood, cannot be achieved with conventional oxygenators.
It is desirable to deliver gas-enriched fluid to a patient in a manner which prevents or minimizes bubble nucleation and formation upon infusion into the patient. The maximum concentration of gas achievable in a liquid is ordinarily governed by Henry's Law. At ambient temperature, the relatively low solubility of many gases, such as oxygen or nitrogen, within a liquid, such as water, produces a low concentration of the gas in the liquid. However, such low concentrations are typically not suitable for treating patients as discussed above. Rather, it is advantageous to use a gas concentration within a liquid that greatly exceeds its solubility at ambient temperature. Compression of a gas and liquid mixture at a high pressure can be used to achieve a high dissolved gas concentration according to Henry's Law, but disturbance of a gas-saturated or a gas-supersaturated liquid by attempts to inject it into an environment at ambient pressure from a high pressure reservoir ordinarily results in cavitation inception at or near the exit port. The rapid evolution of bubbles produced at the exit port vents much of the gas from the liquid, so that a high degree of gas-supersaturation no longer exists in the liquid at ambient pressure outside the high-pressure vessel. In addition, the presence of bubbles in the effluent generates turbulence and impedes the flow of the effluent beyond the exit port. Furthermore, the coalescence of gas bubbles in blood vessels may tend to occlude the vessels and result in a gaseous local embolism that causes a decrease in local circulation, arterial hypoxemia, and systemic hypoxia.
In gas-enriched fluid therapies, such as oxygen therapies involving the use of hyperoxic or hyperbaric blood, delivery techniques are utilized to prevent or minimize the formation of cavitation nuclei so that clinically significant bubbles do not form within a patient's blood vessels. However, it should be understood that any bubbles that are produced tend to be very small in size, so that a perfusionist would typically have difficulty detecting bubble formation without the assistance of a bubble detection device. Unfortunately, known bubble detectors are ineffective for detecting bubbles in an extracorporeal circuit for the preparation and delivery of hyperoxic or hyperbaric blood. This problem results from the fact that the size and velocity of some bubbles are beyond the resolution of known bubble detectors. Therefore, micro bubbles (bubbles with diameters of about 50 micrometers to about 1000 micrometers) and some macro bubbles (bubbles with diameters greater than 1000 micrometers) may escape detection.